1. Field of the Invention
The present invention relates to an apparatus and method for ultrasound imaging of biological tissue according to the impulse-echo technique, and more particularly the present invention relates to a novel and unobvious apparatus and method for reconstructing images of reflection in biological tissue or other media using synthetically focused ultrasound energy.
2. The Prior Art
It has long been known that ultrasonic or acoustic waves in the frequency range of 15,000 cycles per second and higher can be propagated through many solids and liquids. Ultrasound waves are usually considered to be those in the frequency range from approximately 50,000 cycles per second to 10,000,000 cycles per second and higher. Ultrasound energy waves may be partially reflected and partially transmitted at any interface between two media of different density. The product of material density and sonic wave velocity is known as the acoustic impedance, and the amount of reflection which occurs at the interface between two media is dependent upon the amount of change in the acoustic impedance between one medium as opposed to the other medium.
These principles have long been used for imaging reflecting bodies within an ultrasound propagation medium. For example, the organs of a human body as well as bones and sinew act as reflecting bodies within the soft tissue of the body. Likewise, any foreign inclusion will act as a reflecting body. Thus, noninvasive internal examination and medical diagnosis of the human body by ultrasound imaging has long been known in the art. For example, piston type transducers have been used for over 30 years to image some parts of the body.
The inherent advantages of noninvasive medical diagnosis by ultrasound imaging are readily apparent. Unlike exploratory surgery or x-rays, ultrasound imaging permits internal examination of an organ without damaging the surrounding tissue and organs of the body and with much less trauma to the patient.
However, despite the many advantages which may be derived from noninvasive examination by ultrasound imaging, in the past ultrasound imaging has been somewhat limited in its application. One of the primary problems encountered in this regard is the difficulty in providing reflection images of high quality resolution. These images may often be blurred or distorted to some degree, making accurate diagnosis difficult, particularly with respect to very small objects in the body.
Thus, recently much attention has been directed to improving the quality of resolution of ultrasound images. For example, linear phased (or so-called "beam steering") transducer arrays have demonstrated improved depth of focus and time resolution. See, for example, Somer, J. C., W . A. Oosterbaan and H. J. Freund: Ultrasonic Tomographic Imaging of the Brain with an Electronic Sector Scanning System, Proceedings of the 1973 IEEE Ultrasonic Symposium, Nov. 5-7, 1973; and Thurstone, F. L., and O. T. Van Ramm: A New Ultrasound Imaging System Employing Two-Dimensional Electronic Beam Steering, Heart Bulletin, Volume 4, p. 51 (1973). It has also been demonstrated that non-straight line transducer array configurations may be employed to enhance resolution quality in ultrasound images by increasing the aperture for transmitting and receiving ultrasound energy. See, for example, Maginness, M. G., J. D. Plummer and J. D. Meindl: A Cardiac Dynamics Visualization System, Proceedings of the 1973 IEEE Ultrasonic Symposium, Nov. 5-7, 1973; and Green, P. S., L. F. Schaefer, E. D. Jones and J. R. Suarez: A New High-Performance Ultrasonic Camera System, Fifth International Symposium on Acoustical Holography Imaging, July 18-20, 1973. These improvements in the resolution quality of ultrasound images increase the ability of a system to detect small, focal lesions such as cancer, abscesses, or infarcts of less than one centimeter in diameter.
However, often more fundamental than the focal lesion itself, regardless of its size, is the state of the tissue surrounding the lesion. The pattern of the adjacent tissue plays a crucial role in the identification of specific diseases by supplying the physician with information concerning the local context of the bodily processes which are resulting in the lesion.
Compared to tumor nodules and the like, the structures of the surrounding normal tissue are relatively delicate. The tissue structure is essentially determined by the dimensions of the fibrous and vascular framework of an organ and is responsive to the pathologic processes.
It would therefore be highly desirable to be able to image such delicate patterns as those associated with, for example, the interstitial spaces of parenchymal organs, or the tertiary branching of major arteries of the heart, brain, kidneys and lungs, or the biliary ducts of the liver and the ductular system of the breast and pancreas. However, ultrasound imaging of these delicate tissues requires a resolution capability not presently possible with commercially known ultrasound scanning systems.
Accordingly, what is needed is an improved ultrasound imaging apparatus and method capable of high quality resolution for images of reflection for highly delicate tissue and the like. Such a device would provide a significant advancement in the state of the art by providing noninvasive diagnostic techniques through ultrasound imaging which could be used for pathogenesis, prevention and early detection of disease rather than being limited to the diagnosis of gross advanced lesions. Such an invention is disclosed and claimed herein.